My first thought was: why have I never heard of this? The answer seems to be: 2" x 3" photo paper costs $0.41/ea.
Mail-order professional photo prints are quite a bit cheaper than that. So, the main thing this contends with is other instant photo creation methods. Which are pretty niche ever since digital cameras became good.
Absolutely anything to avoid the metric system! If we, as Americans, has to measure in quotation marks, we damn sure will!